In materials used in an automotive field, a household appliance field, and a building material field, a surface treated steel sheet which is imparted with corrosion prevention is being used. In particular, an alloyed hot-dip galvanized steel sheet which can be produced at low cost and is excellent in corrosion prevention is being used.
In general, the alloyed hot-dip galvanized steel sheet is manufactured by the following method using a continuous hot-dip galvanizing plant. First, a slab is hot rolled, cold rolled, or heat treated to obtain a thin-gauge steel sheet. The thin-gauge steel sheet is degreased and/or pickled in a pretreatment step for the purpose of cleaning the surface of the base steel sheet or, omitting the pretreatment step, is heated in a preheating furnace to burn off the oil on the surface of the base steel sheet, then is subjected to heating and recrystallization annealing. The atmosphere at the time of performing the recrystallization annealing is an Fe reducing atmosphere since at the time of the later plating treatment, Fe oxides would obstruct the wettability of the plated layer and the base steel sheet or the adhesion of the plated layer and the base steel sheet. After the recrystallization annealing, without contacting the air, the steel sheet is continuously cooled to a temperature suitable for plating in an Fe reducing atmosphere and dipped in a hot-dip galvanizing bath for hot-dip galvanization. After the hot-dip galvanization, the amount of adhesion of the plating is controlled by immediately performing wiping by nitrogen gas. After that, the heating is performed to thereby conduct an Fe—Zn alloying reaction, and in this way, the alloyed hot-dip galvanized layer is formed on the base steel sheet.
In recent years, in particular in the automotive field, to achieve both the function of protecting the passengers at the time of collision and lighter weight aimed at improvement of the fuel efficiency, use of a high-strength steel sheet which is made higher in strength of the base steel sheet by inclusion of elements which are relatively inexpensive, such as C, Si, and Mn, has been increasing. Regarding the strength, the steel sheet having a tensile strength of 590 MPa or more is mainly used.
However, in the high-strength alloyed hot-dip galvanized steel sheet including Si and Mn, Si and Mn are elements which are more easily oxidizable compared with Fe, so at the time of heating in recrystallization annealing in a conventional Fe-reducing atmosphere, Si and Mn on the surface of the steel sheet oxidize. Further, Si and Mn which thermally diffuse from the inside of the steel sheet oxidize at the steel sheet surface whereby gradually the Si and Mn oxides become concentrated on the surface. If the Si and Mn oxides concentrate at the surface, in the process of dipping the steel sheet in the hot-dip galvanizing bath, contact between the molten zinc and the base steel sheet would be prevented, which would cause a drop in the wettability of plating and the adhesion of plated layer of the alloyed hot-dip galvanized layer. If the plating layer deteriorates in wettability, nonplating defects occur and result in defects in appearance and defects in corrosion prevention. If the adhesion of plated layer deteriorates, peeling of the plating occurs when press forming is performed, and results in problems including defects in corrosion prevention and defects in appearance with press scratches and the like.
Further, in the high-strength alloyed hot-dip galvanized steel sheet containing C, when C is present in a grain boundary or a grain of the base steel sheet in the recrystallization annealing, there is a problem in that the reaction between the molten zinc and the steel sheet in the process of Fe—Zn alloying reaction after dipping the base steel sheet in the hot-dip galvanizing bath is inhibited, to thereby deteriorate the adhesion of plated layer. In addition, there is also a problem in that the inclusion of C in the alloyed hot-dip galvanized layer after the alloying reaction lowers the ductility of the plating, so that peeling of the plating easily occurs when press forming is performed.
Still further, in the high-strength alloyed hot-dip galvanized steel sheet, the ductility deteriorates with the increase in the strength of the base steel sheet, and along therewith, pressing load at the time of performing press forming is large, so that the shear stress applied to the plated layer from a mold at the time of performing forming increases. Accordingly, there is a problem that the plated layer is easily peeled from the interface with the base steel sheet, and results in problems including defects in corrosion prevention and defects in appearance with press scratches and the like.
As measures for the problems attributed to the concentration of oxides of Si and Mn at the time of annealing, there have been proposed various techniques in the past.
As the technique focusing on suppressing concentration of oxides of Si and Mn, Patent Literature 1 shows a method including performing annealing under an oxidizing atmosphere of Si so that the thickness of the oxide film of the steel sheet surface becomes 400 to 10000 Å, then reducing the Fe in the furnace atmosphere containing hydrogen, and performing plating. Further, Patent Literature 2 shows a method including oxidizing the Fe on the steel sheet surface, controlling the oxygen potential in the reducing furnace to thereby reduce the Fe and internally oxidize the Si so as to suppress the concentration of Si oxides on the surface, and then performing plating. However, in those techniques, if the reduction time is too long, Si concentrates at the surface, and if the reduction time is too short, an Fe oxide film remains on the steel sheet surface. Accordingly, there is the problem that issues in the plating layer wettability and the plating layer adhesion are insufficiently resolved. In addition, if Fe oxides are formed on the steel sheet surface inside an annealing furnace, the Fe oxides are deposited on a roll inside the furnace, and with increase in the amount of the deposit, there is a problem that roll pickup is caused, such as defects in appearance with press scratches on the steel sheet.
Patent Literature 3 shows a technique of suppressing the concentration of oxides of Si and Mn on the surface by raising the oxygen potential in the atmosphere in an all radiant tube type annealing furnace and internally oxidizing Si and Mn. Further, Patent Literatures 4 and 5 show methods including carefully controlling the means and conditions for raising the oxygen potential to suppress the surface concentration of both Fe oxides and Si and Mn oxides, and then performing plating. However, none of those techniques are insufficient in suppressing the concentration of oxides of Si and Mn. Further, since internal oxides of Si and Mn formed on the surface of the base steel sheet are present in the vicinity of the surface of the inside of the base steel sheet, there is a problem that the ductility of the base steel sheet deteriorates and the press forming cannot be performed. In addition, when a shear stress is applied to the plated layer at the time of performing the press forming, there is a problem that the plated layer peels from the vicinity of the surface of the inside of the base steel sheet in which the internal oxides are present.
Patent Literature 6 shows a method including raising the hydrogen concentration in the atmosphere in the recrystallization annealing up to the reducing region in which Fe, Si, and Mn do not oxidize, and performing plating. However, in this technique, there is a problem in addition to that the cost of hydrogen becomes immense, that the presence of C on the surface of the base steel sheet deteriorates the adhesion of plated layer as described above, and the remaining Si and Mn obstruct the reaction between the plating and the base steel sheet and form oxides of Si and Mn by being reacted with oxides floating on the surface of the bath at the time of dipping in the plating bath, so the wettability of plating and the adhesion of plated layer deteriorate.
Further, as a technique for suppressing the concentration of oxides of Si and Mn, Patent Literature 7, which focuses on causing internal oxidation in advance in the hot rolling step, shows a technique of controlling the oxygen potential in the hot rolling step so as to cause internal oxidation of Si and using the resultant thin-gauge steel sheet to manufacture a hot-dip galvanized steel sheet in a continuous hot-dip galvanizing plant. However, in this technique, at the time of the cold rolling step and other rolling, the layer of internal oxidation also ends up being rolled together, so the internal oxidation layer becomes smaller in thickness and Si oxides end up concentrating on the surface in the recrystallization annealing process, so there is a problem that the wettability of plating and the adhesion of plated layer are insufficiently improved. Further, there is a problem that oxides of Fe, which are formed simultaneously with internal oxidization of Si in the hot rolling step, cause roll pickup.
Further, the techniques written in Patent Literatures 1 to 7 are insufficient for solving the problem of the adhesion of plated layer related to the deterioration of ductility caused by increase in the strength of the alloyed hot-dip galvanized steel sheet.